The precise shape and size of an electric toothbrush's brush head will vary according to brand and model. For instance, a grown adult will be able to accommodate the use of a larger brush head than a twelve year old. Additionally, those with braces will probably find that a generally smaller brush head is easier to use than a larger one. All you really need to do is evaluate your family's needs and proceed in finding brush heads that are available for the electric toothbrush you are considering. You could very well find that the model you've chosen doesn't come packaged with the size brush head you prefer, but the better brush head can be bought as an accessory.